When I turn off my MacBook Pro, I get a purple screen.

Personally, I'd schedule an appointment at my Apple Store Genius Bar and let Apple see it.

Is that permanent or temporary?

From the moment you choose "shutdown" onward, what you see on the display is "undefined".

Apple will not be accepting any bug reports based on your dislike for "undefined".

Ignore it until, or unless, it becomes and actual problem.

Good luck.